Another way to facilitate the access of the mixture to the spark was proposed by Japanese constructors. They introduced a groove in one of the electrodes. At NGK, this V-shaped groove is made on the face of the center electrode. DENSO's spark plugs, on the other hand, have a U-shaped inner groove in the side electrode as standard, which creates additional space for the spark to fill. As a result, the flame nucleus has more space to spread, resulting in a larger and hotter fire front. The result is more regular engine operation, especially adapted to the combustion of leaner mixtures. The spark has a choice of one of four paths between the copper-core center electrode and the edges of the groove. The jump of the spark always takes place to the highest and closest point. The "U" groove creates the effect of an increased spark gap, although the nominal electrode gap is maintained.
In a similar "U" groove, but with smaller dimensions, the designers from DENSO equipped their ZU platinum candle. The center platinum electrode is in diameter 0,7 mm, and the grooved side electrode has a pointed end. The advantage of using a thin center electrode is the low ignition voltage requirement and the generation of a strong spark. The ZU spark plug facilitates engine start-up and is recommended for high-power and high-performance engines. The manufacturer specifies its durability on 100 000 km.
Autolite Double Platinum candles achieve the same durability, thanks to the use of platinum tips on both the middle and side electrodes. Platinum, as a high-strength material, excellent conductivity and wear-resistant, ensures long life of the plug and reliable ignition of the mixture sucked into the cylinders. This enables optimum engine power and lower fuel consumption to be obtained.